Bioimpedance measurement system and method

ABSTRACT

Methods and systems for detecting fluid egress, assessing lesion quality, determining tissue composition or structure, determining ice coverage of catheter tip and providing tissue contact assessment, by providing a catheter having a shaft with a proximal end portion and a distal end portion, the proximal end portion and the distal end portion define at least one fluid pathway therebetween with the shaft having a plurality of electrodes, positioning the catheter at a tissue treatment site, applying an electrical current between at least two of the plurality of electrodes, measuring impedance voltage between the at least two of the plurality of electrodes and, processing the measured impedance voltage caused by the applied electrical current to determine if fluid egress is present, to assess lesion quality, to determine tissue composition, ice cover of catheter tip, and to provide contact assessment. The system may have a control unit, a microprocessor, an impedance measuring device or the like to perform processing of impedance data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. Utility patent application Ser.No. 11/283,057, filed Nov. 18, 2005, entitled BIOIMPEDANCE MEASUREMENTSYSTEM AND METHOD, the entirety of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

FIELD OF THE INVENTION

The present invention relates to a system and method for treating tissueusing cooled medical devices using electrical impedance measurementswith the device.

BACKGROUND OF THE INVENTION

Effectiveness of cryotreatment of endocardial tissue is significantlyaffected by the contact of the catheter tip or thermally transmissiveregion to the tissue. Ex-vivo studies show a correlation between thelesion sizes created and the tip or thermally-transmissive region totissue contact quality. A larger lesion size can be achieved with thesame device by improving the tip to tissue pressure or contact. Variousmethods have been used to assess tip or thermally-transmissive regioncontact, using RF catheters and/or ultrasound imaging. However, none ofthese methods has proved entirely satisfactory.

The problem extends to all areas of tissue treatment wherein the tissueundergoes some change or “physiological event” during the course oftreatment. In addition to contact quality assessment, in treatmentdevices that employ fluid flows, detection and containment of leaks is acritical problem, especially in the operation of cryogenic devices fortherapeutic purposes, lest a leak of coolant enter the body and therebycause significant harm. Known catheters which employ inflatable balloonsoften inflate the balloons to relatively high pressures that exceed theambient pressure in a blood vessel or body lumen. However, to containthe coolant, these catheters generally employ thicker balloons,dual-layered balloons, mechanically rigid cooling chambers, and othersimilar unitary construction containment mechanisms. These techniqueshowever, lack robustness, in that if the unitary balloon, coolingchamber, or other form of containment develops a crack, leak, rupture,or other critical structural integrity failure, coolant may egress fromthe catheter. To minimize the amount and duration of any such leaks, itis desirable to use a fluid detection system that detects a gas orliquid expulsion or egress from the catheter shaft and signals a controlunit to halt the flow of cryogenic fluid.

Furthermore, since many treatment systems and methods are applied ininternal body lumens, organs or other unobservable tissue regions, theorientation and attitude of the device structure relative to the tissueis of significant importance in ensuring the effective and efficienttreatment of tissue. This applies to many tissue treatment systems, bothsurgical and non-surgical, using a variety of modalities, includingcooling through cryotreatment, heat or electrically induced heating,ultrasound, microwave, and RF, to name a few.

This collection of problems may be resolved in part by developing aspecialized transducer suitable for the “body” environment in which itoperates. For many physiological events, there is no specializedtransducer. The events in question include changes in the natural stateof tissue, such as temperature, dielectric or conductivity changes,structural changes to the cells and cell matrix, dimensional changes, orchanges in the operation of, or interplay between, tissue regions and/orforeign bodies, such as blood flow in an artery having a treatmentdevice inserted therein.

All of these changes may be correlated to, or affected by, relativechanges in the bioelectrical impedance of the tissue region.

It would be desirable to provide an apparatus and method of assessinglesion quality, monitoring and detecting any occurrences of fluidegress, determining blood vessel occlusion, determining tissuecomposition as well as assessing the quality of the contact between thetip or thermally-transmissive region of a cryogenic device and thetissue to be treated.

SUMMARY OF THE INVENTION

The present invention advantageously provides methods and systems fordetecting fluid egress, assessing lesion quality, determining tissuecomposition or structure, and providing tissue contact assessment.

In an exemplary embodiment, a method is provided for detecting fluidegress including the steps of positioning a catheter at a tissuetreatment site, where the catheter includes a shaft, which has aproximal end portion and a distal end portion, wherein the proximal endportion and the distal end portion define at least one fluid pathwaybetween the distal end portion and the proximal end portion, and theshaft has a plurality of electrodes, applying an electrical currentbetween at least two of the plurality of electrodes, measuring impedancevoltage between the at least two of the plurality of electrodes and,processing the measured impedance voltage resulting from the appliedelectrical current to determine if fluid egress is present.

The processing step of the method for detecting fluid egress may includethe steps of establishing a normal impedance voltage range, monitoringto determine if the impedance voltage varies outside of the impedancevoltage range, and generating a signal if the impedance voltagemeasurement varies outside of the impedance voltage range. A controlunit, a microprocessor, an impedance-measuring device or the like mayperform the processing step. In another embodiment of the method, thetreatment portion of catheter may include a cooling chamber in fluidcommunication with the at least one fluid pathway and having the firstelectrode located near the distal side of the cooling chamber, and thesecond electrode located near the proximal side of the cooling chamber.

In another exemplary embodiment, a method is provided for accessinglesion quality including the steps of positioning a catheter at a tissuetreatment site, where the catheter includes a shaft, which has aproximal end portion and a distal end portion, wherein the proximal endportion and the distal end portion define at least one fluid pathwaytherebetween, and the shaft has a treatment portion that includes afirst electrode and a second electrode, and measuring a baselineimpedance, activating the catheter such that the treatment portion coolsthe tissue, applying an electrical current between the first and secondelectrodes, and processing the measured impedance voltage caused by theapplied electrical current to determine the amount of treated tissueafter each activation of the catheter.

The processing step of the method for accessing lesion quality may beperformed by a control unit, a microprocessor, an impedance measuringdevice or the like. In another embodiment of the method, the treatmentportion of catheter may include a cooling chamber in fluid communicationwith the at least one fluid pathway and having the first electrodelocated near the distal side of the cooling chamber, and the secondelectrode located near the proximal side of the cooling chamber.

In still another exemplary embodiment, a method is provided foraccessing tissue composition including the steps of positioning acatheter at a tissue treatment site, where the catheter includes ashaft, which has a proximal end portion and a distal end portion,wherein the proximal end portion and the distal end portion define atleast one fluid pathway therebetween, and the shaft has a treatmentportion that includes a first electrode and a second electrode,activating the catheter such that the treatment portion cools thetissue, applying an electrical current between the first and secondelectrodes, measuring a impedance voltage between the first and secondelectrodes, and processing the measured impedance caused by the appliedelectrical current, establishing a normal impedance range for a tissuetype, monitoring the impedance to determine if the impedance varies intoa tissue type impedance range, and generating an impedance signal thatcan be processed to identify the tissue type impedance range.

The processing step of the method for accessing tissue composition maybe performed by a control unit, a microprocessor, an impedance measuringdevice or the like. In another embodiment of the method, the treatmentportion of catheter may include a cooling chamber in fluid communicationwith the at least one fluid pathway and having the first electrodelocated near the distal side of the cooling chamber, and the secondelectrode located near the proximal side of the cooling chamber.

In still another exemplary embodiment, a method is provided foraccessing tissue composition including the steps of positioning acatheter at a tissue treatment site, where the catheter includes ashaft, which has a proximal end portion and a distal end portion,wherein the proximal end portion and the distal end portion define atleast one fluid pathway therebetween, and the shaft has a treatmentportion that includes a first electrode and a second electrode,activating the catheter such that the treatment portion cools thetissue, applying an electrical current between the first and secondelectrodes, measuring a impedance voltage between the first and secondelectrodes, and processing the measured impedance caused by the appliedelectrical current, delivering coolant to the treatment tip, measuring asecond impedance voltage between the first and second electrodes,processing the measured impedance voltage caused by the appliedelectrical current to determine a delta impedance from the first andsecond impedances, and determining if the delta impedance has reached amaximum value.

The processing step of the method for accessing tissue composition maybe performed by a control unit, a microprocessor, an impedance measuringdevice or the like. In another embodiment of the method, the treatmentportion of catheter may include a cooling chamber in fluid communicationwith the at least one fluid pathway and having the first electrodelocated near the distal side of the cooling chamber, and the secondelectrode located near the proximal side of the cooling chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates a catheter system having an expandablethermally-transmissive region;

FIG. 2 illustrates an embodiment of a shaft of the catheter system ofFIG. 1;

FIG. 2A illustrates an embodiment of the catheter system used with apair of excitation electrodes positioned on a patient's body;

FIG. 3 illustrates a catheter system having a non-expandablethermally-transmissive region;

FIG. 3A illustrates a catheter system having measurement electrodes onthe inside of a guidewire lumen;

FIG. 3B illustrates a cutaway view of the guidewire lumen of FIG. 3A;

FIG. 4A illustrates an embodiment of a catheter in a deflectedconfiguration and positioned near a treatment site;

FIG. 4B illustrates an embodiment of the catheter tip of FIG. 4A havingfour electrodes in its thermally-transmissive region;

FIG. 4C illustrates an embodiment the catheter tip of FIG. 4A havingeight electrodes in its thermally-transmissive region;

FIG. 5 illustrates an embodiment of a catheter for detecting fluidegress from a catheter;

FIG. 6 illustrates an embodiment of a fluid egress algorithm;

FIG. 7 illustrates an embodiment of a pulmonary vein occlusion/fluidegress/ice coverage verification algorithm;

FIG. 8 illustrates an embodiment of a ice coverage/lesion qualitymeasurement algorithm;

FIG. 9 illustrates a graph of the general impedance Z(t) with respect totime; and

FIG. 10 illustrates a graph of the general impedance Z(t) with animpedance spike due to a fluid egress.

DETAILED DESCRIPTION OF THE INVENTION

A system and method for detecting fluid egress including the steps ofpositioning a catheter at a tissue treatment site, where the catheterincludes a shaft, which has a proximal end portion and a distal endportion, wherein the proximal end portion and the distal end portiondefine at least one fluid pathway therebetween, and the shaft has atreatment portion that includes at least four electrodes, a first pairof electrodes and a second pair of electrodes, applying an electricalcurrent between the first pair of electrodes, measuring a impedancevoltage between the second pair of electrodes, and processing themeasured impedance voltage caused by the applied electrical current todetermine if fluid egress is present.

The number and location of the electrodes will affect the systemmeasurement sensitivity. For example, as the distance between the pairof measurement electrodes is decreased, the system measurementsensitivity is increased. On the other hand, as the distance between thepair of measurement electrodes and the pair of excitation currentelectrodes is decreased, the system measurement sensitivity isdecreased. In another embodiment, where a catheter having a single pairof electrodes for both measuring impedance and providing the excitationcurrent, the system measurement sensitivity is also decreased.

The processing step of the method for detecting fluid egress may includethe steps of establishing a normal impedance voltage range, monitoringto determine if the impedance voltage varies outside of the impedancevoltage range, and generating a signal if the impedance voltagemeasurement varies outside of the impedance voltage range. A controlunit, a microprocessor, an impedance-measuring device or the like mayperform the processing step. In another embodiment of the method, thetreatment portion of the catheter may include a cooling chamber in fluidcommunication with the at least one fluid pathway and having one of eachpair of electrodes located near the distal side of the cooling chamber,and one of each pair of electrodes located near the proximal side of thecooling chamber.

In addition, a system and method for accessing lesion quality includingthe steps of positioning a catheter at a tissue treatment site, wherethe catheter includes a shaft, which has a proximal end portion and adistal end portion, wherein the proximal end portion and the distal endportion define at least one fluid pathway therebetween, and the shafthas a treatment portion that includes a first electrode and a secondelectrode, activating the catheter such that the treatment portion coolsthe tissue, applying an electrical current between the first and secondelectrodes, measuring a impedance voltage between the first and secondelectrodes, and processing the measured impedance voltage caused by theapplied electrical current to determine the amount of treated tissueafter each activation of the catheter.

The processing step of the method for accessing lesion quality may beperformed by a control unit, a microprocessor, an impedance measuringdevice or the like. In another embodiment of the method, the treatmentportion of catheter may include a cooling chamber in fluid communicationwith the at least one fluid pathway and having the first electrodelocated near the distal side of the cooling chamber, and the secondelectrode located near the proximal side of the cooling chamber.

In addition, a system and method for accessing tissue compositionincluding the steps of positioning a catheter at a tissue treatmentsite, where the catheter includes a shaft, which has a proximal endportion and a distal end portion, wherein the proximal end portion andthe distal end portion define at least one fluid pathway therebetween,and the shaft has a treatment portion that includes a first electrodeand a second electrode, activating the catheter such that the treatmentportion cools the tissue, applying an electrical current between thefirst and second electrodes, measuring a impedance voltage between thefirst and second electrodes, and processing the measured impedancecaused by the applied electrical current, establishing a normalimpedance range for a tissue type, monitoring the impedance to determineif the impedance varies into a tissue type impedance range, andgenerating a signal when the impedance varies into the identified tissuetype impedance range.

The processing step of the method for accessing tissue composition maybe performed by a control unit, a microprocessor, an impedance measuringdevice or the like. In another embodiment of the method, the treatmentportion of catheter may include a cooling chamber in fluid communicationwith the at least one fluid pathway and having the first electrodelocated near the distal side of the cooling chamber, and the secondelectrode located near the proximal side of the cooling chamber.

As many treatment systems and methods are applied in internal bodylumens, organs or other unobservable tissue regions, the orientation andattitude of the device structure relative to the tissue is ofsignificant importance in ensuring the effective and efficient treatmentof tissue. This applies to many tissue treatment systems, both surgicaland non-surgical, using a variety of modalities, including coolingthrough cryotreatment, heat or electrically induced heating, ultrasound,microwave, and RF, to name a few.

Many of these events include changes in the natural state of tissue,such as temperature, dielectric or conductivity changes, structuralchanges to the cells and cell matrix, dimensional changes, or changes inthe operation of, or interplay between, tissue regions and/or foreignbodies, such as blood flow in an artery having a treatment deviceinserted therein. All of these changes may be correlated to, or affectedby, relative changes in the bio-impedance of the tissue region.

When using the term impedance, we are referring to the generallyaccepted definition of the term: a complex ratio of sinusoidal voltageto current in an electric circuit or component, except that as usedherein, impedance shall apply to any region or space through which someelectrical field is applied and current flows. The impedance, Z, may beexpressed as a complex number, Z=R+jX, wherein R is the resistance inreal number ohms, X is the reactance in imaginary number ohms, and j isa multiplier that is the positive square root of negative one (−1).

Resistance, denoted R, is a measure of the extent to which a substanceopposes the movement of electrons among its atoms. The more easily theatoms give up and/or accept electrons, the lower the resistance.Reactance denoted X, is an expression of the extent to which anelectronic component, circuit, or system stores and releases energy asthe current and voltage fluctuate with each AC (alternating current)cycle. Reactance is expressed in imaginary number ohms. It is observedfor AC, but not for DC (direct current). When AC passes through acomponent that contains reactance, energy might be stored and releasedin the form of a magnetic field, in which case the reactance isinductive (denoted +jX_(L)); or energy might be stored and released inthe form of an electric field, in which case the reactance is capacitive(denoted −jX_(C)). The impedance Z may be positive or negative dependingon whether the phase of the current lags or leads on that of thevoltage. Impedance, sometimes called “apparent resistance”, is differentfrom general resistance, in that impedance applies only to AC; howeverresistance R applies to both AC and DC, and is expressed in positivereal number ohms.

As mentioned in the background section, the collection of problems maybe resolved in part by developing a specialized transducer suitable forthe “body” environment in which it operates. However, for manyphysiological events, there is no specialized transducer. The events inquestion include changes in the natural state of tissue, such astemperature, dielectric or conductivity changes, structural changes tothe cells and cell matrix, dimensional changes, or changes in theoperation of, or interplay between, tissue regions and/or foreignbodies, such as blood flow in an artery having a treatment deviceinserted therein. Using simple transducers, such as electrodes properlypositioned in the tissue, the impedance between them is measured, whichmay depend on seasonal variations, blood flow, cardiac activity,respired volume, nervous activity, galvanic skin reflex, blood pressure,and salivation, to name a few. In some cases the measured impedance maybe dissected into its resistive and reactive components. In other casesthe total impedance may be measured, with or without resolution into itscomponents, which may contain sufficient information on thephysiological event, especially when measured relative to some referenceor “baseline” impedance prior to the physiological event.

Additionally, during the operation of a medical device in a therapeuticprocedure, such as in a blood vessel, the heart or other body organ, themedical user desires to establish a stable and uniform contact betweenthe tip or thermally-transmissive region of the cryogenic device and thetissue to be treated (e.g., ablated). In those instances where thecontact between the tip or thermally-transmissive region of thecryogenic device and the tissue to be treated is non-uniform orinstable, the resulting ablation or lesion may be less than optimal. Itis desirable for the medical professional to assess the state of thecontact between the tip or thermally-transmissive region of thecryogenic device and the tissue to be treated, so that appropriateadjustments can be made to re-position the cryogenic device to obtain amore optimal contact and thus a more effective treatment.

In view of the proceeding, the present invention advantageously providesmethods and systems for detecting fluid egress, assessing lesionquality, determining tissue composition and structure, determining icecoverage of catheter tip as well as providing tissue contact assessment.

FIG. 1 illustrates an exemplary system 30 for performing cryogenicablation. The system 30 includes an elongate, highly flexible ablationcatheter 34 that is suitable for passage through the vasculature. Theablation catheter 34 includes a catheter body 36 having a distal end 37with a thermally conductive region 38 at or proximal to the distal end37. The distal end 37 and the thermally conductive region 38 are shownmagnified and are described in greater detail below. The catheter body36 has a proximal end 40 that is mated to a handle 42 that can includean element such as a lever 44 or knob for manipulating the catheter body36 and the thermally conductive region 38. In the exemplary embodiment,a pull wire 46 with a proximal end and a distal end has its distal endanchored to the catheter at or near the distal end 37. The proximal endof the pull wire 46 is anchored to an element such as a cam 48 incommunication with and responsive to the lever 44. The handle 42 canfurther include circuitry 50 for identification and/or use incontrolling of the ablation catheter 34 or another component of thesystem 30.

Continuing to refer to FIG. 1, the handle 42 can also include connectorsthat are matable directly to a cryogenic fluid supply/exhaust andcontrol unit or indirectly by way of one or more umbilicals. In thesystem illustrated, the handle 42 is provided with a first connector 54that is matable with a co-axial fluid umbilical (not shown) and a secondconnector 56 that is matable with an electrical umbilical (not shown)that can further include an accessory box (not shown). In the exemplarysystem the fluid supply and exhaust, as well as various controlmechanisms for the system are housed in a single console 52. In additionto providing an exhaust function for the ablation catheter fluid supply,the console 52 can also recover and/or re-circulate the cooling fluid.The handle 42 is provided with a fitting 58 for receiving a guide wire(not shown) that is passed into a guide wire lumen 60. During ballooninflation, contrast solution may be injected through the catheter'sinner guide wire lumen 60 and into the pulmonary vein.

Still referring to FIG. 1, the thermally conductive region 38 is shownas a double balloon having a first membrane (e.g., inner balloon) 62contained or enclosed within a second membrane (e.g., outer balloon) 64,thereby defining an interface or junction 57 between the first andsecond membranes. The second membrane 64 provides a safeguard to preventfluid from leaking out of the cooling chamber 55 and into surroundingtissue should the first membrane 62, and therefore the cooling chamber55, rupture or develop a leak. The junction 57 between the first andsecond membranes 62, 64 may be substantially under a vacuum, such thatthe first and second membranes 62, 64 are generally in contact with eachother, with little or no open space between them. A coolant supply tube66 in fluid communication with the coolant supply in the console 52 isprovided to release coolant from one or more openings in the tube withinthe inner balloon 62 in response to console commands and other controlinput. A vacuum pump in the console 52 creates a low-pressureenvironment in one or more lumens within the catheter body 36 so thatcoolant is drawn into the lumen(s), away from the inner balloon 62, andtowards the proximal end of the catheter body. The vacuum pump is alsoin fluid communication with the interface or junction 57 of the innerand the outer balloons 62, 64 so that any fluid that leaks from theinner balloon 62 is contained and aspirated.

Still referring to FIG. 1, the handle 42 includes one or more pressuresensors 68 to monitor the fluid pressure within one or both of theballoons, the blood detection devices 70 and the pressure relief valves72. When coolant is released into the inner balloon 62, the inner andthe outer balloon 64 expand to a predetermined shape to present anablation surface, wherein the temperature of the ablation surface isdetermined by the material properties of the specific coolant selectedfor use, such as nitrous oxide, along with the pressure within the innerballoon 62 and the coolant flow rate.

FIG. 2 illustrates an embodiment of a shaft or catheter body 36 of theballoon catheter system 34 of FIG. 1. The catheter body 36 includes amounting section 59 in communication with the proximal end of thermallyconductive element 38. The inner balloon 62 and outer balloon 64 arebonded to the mounting section 59. In this embodiment, the inner balloon62 and outer balloon 64 are bonded at different locations, which aredefined as the inner balloon bond joint 63 and the outer bond joint 65,however they may be bonded at the same bond joint. Additionally, severalsensors are identified including a temperature sensor 61 (e.g.,thermocouple wire), leak detectors 67, 69 (e.g., leak detection wires)and electrodes 86, 88, 90 and 92. In this embodiment, contactassessment, lesion quality, fluid egress and/or tip ice coverage may beprovided by using a first pair of electrodes (86, 88); providing theexcitation current 107 of well-selected amplitude (e.g., in the range of0.2 mA to 5 mA) and frequency (e.g., in the range of 250 Hz to 500 kHz)to create a current field and measuring the differential impedancevoltage as produced across a second pair of electrodes (90, 92).

In another embodiment of the catheter having a single pair of electrodes(e.g., 90 and 92) with one of the pair of electrodes located on thedistal side of the thermally-transmissive region 38 (e.g., a singleballoon), and the other of the pair located on the proximal side of thethermally-transmissive region 38, an excitation current 107 ofwell-selected amplitude and frequency is applied between the twoelectrodes to create a current field and measure the differentialimpedance voltage as produced across the same electrodes to determinetissue contact assessment, lesion quality and/or blood occlusionassessment. The processing algorithms and related aspects will bediscussed in more detail below.

In another embodiment, as illustrated in FIG. 2A, a pair of excitationcurrent electrodes (86, 88) are located on a patient's body and createan electrical field (i.e., polarize the patient's body), and the pair ofmeasurement electrodes (90, 92) are located on the catheter 34. Thetissue contact assessment, lesion quality and fluid egress aspects canbe determined by applying an excitation current 107 of a well-selectedamplitude and frequency to create a current field and measuring thedifferential impedance voltage as produced across the pair of electrodes(90, 92).

After applying an excitation current to the two electrodes 90, 92, theimpedance voltage can be measured by the impedance measurement system106 (as shown in FIG. 3). The impedance measurement signal is thenprocessed using a signal processor 108 (as shown in FIG. 3), whichextracts relevant data from a specific frequency range to correlate theimpedance change to, for example, occlusion of a pulmonary vein. Thesignal processor 108 may be a standalone unit or it may be part of thecontrol unit 52, the impedance measurement system 106 or another portionof the catheter system. The electrical impedance of the tissue is muchhigher than the impedance of the blood, so measuring the impedancebetween the first electrode 90 and the second electrode 92 wouldindicate the efficacy of a thermally conductive element to tissuecontact. With high measurement sensitivity, the system should be able toquantify the contact quality. The impedance measurement system 106provides information about the baseline impedance that may change as theballoon 38 occludes a vessel, such as a pulmonary vein (PV). As theballoon will occlude or stop the blood flow between the proximal sideand the distal side of the balloon, the impedance at a defined frequencywill increase, which provides an indication of the quality of thecontact between the balloon 38 and the treatment tissue.

FIG. 3 illustrates another embodiment of the thermally conductive region38 of catheter 34 positioned near a treatment tissue site, such as theheart. In this embodiment the thermally conductive region 38 is shown ashaving a thermally conductive non-balloon element. Although thisembodiment is shown with a single thermally conductive element (e.g.,tip 94), the thermally conductive region 38 may have two or morethermally conductive elements. An excitation current of well-selectedamplitude and frequency is applied to the electrodes 90, 92 and tipelectrode 94; the impedance (voltages) can be measured by the impedancemeasurement system 106 (e.g., between the tip electrode 94 and theelectrode 90). Once the excitation current is applied to the electrodesit will produce the electrical current lines 102, which indicate overallfield strength. The excitation field provides for or enables thepolarization of the tissue or treatment area of the patient. The shapeand density of the current lines 102 will characteristically result fromthe number and placement of the electrodes. The number and placement ofthe electrodes will determine the overall system sensitivity. Normally,a greater sensitivity is required to perform fluid egress detection asopposed to tissue contact assessment or tip ice formation.

In general, the detection of catheter fluid egress and of cathetertissue contact assessment may be determined using the same catheter andelectrode configurations. The process for determining fluid egress andtissue contact assessment, typically may be determine by the selectionof the excitation current applied to the catheter system. For example,if a gas bubble leak occurs in the catheter, a low frequency excitationcurrent (e.g., in the range of 250 Hz to 100 kHz) can improve thedetection of the gas bubble leak since the low frequency signal will notpenetrate the gas bubble and the gas bubble will interrupt theelectrical lines 102, and thus cause a spike in the measured impedanceZ. On the other hand, if a high frequency excitation current (e.g., inthe range of 20 kHz to 500 kHz) is applied, the high frequencyexcitation current will penetrate the gas bubble and therefore the gasbubble will not interrupt the electrical lines 102, causing the bubbleto go undetected. Therefore, there are certain circumstances whereadditional electrodes may be necessary to improve the sensitivity of theoverall detection system and process to improve leak detection/fluidegress.

FIGS. 3A and 3B illustrate another embodiment of the thermallyconductive region 38 of catheter 34. In this embodiment, the location ofthe measurement electrodes (87 and 91) is inside the guide wire lumen60. In this embodiment, the inner member 62 and outer member 64 areconnected to the catheter shaft 36 and define the cooling chamber 55. Bylocating the, measurement electrodes (87 and 91) on the inside of guidewire lumen 60, a fluid leak 101 from the catheter shaft 36 or guide wirelumen 60 may be detected.

FIG. 4A illustrates another embodiment of the thermally conductiveregion 38 of catheter 34 positioned near a treatment tissue site, suchas in a pulmonary vein. In this embodiment the thermally conductiveregion 38 is shown as having a thermally conductive non-balloon elementwith a plurality of electrodes 90, 91, 92, 94, etc., wherein thethermally conductive region 38 is in a spiral or coiled configuration.Each of the plurality of electrodes may be monitored by the impedancemeasurement system 106, which can provide information about eachelectrode's baseline impedance that will vary as the thermallyconductive region 38 contacts the targeted treatment tissue. Theimpedance measurement system 106 may use an impedance multiplexer tomeasure the impedance (voltages) between the electrodes of the thermallyconductive region 38 by scanning the electrodes and recording all theimpedance voltages. For example, an excitation current may be appliedbetween electrodes 94 and 92 and the impedance voltage may be measuredbetween electrodes 90 and 91. Next an excitation current may be appliedbetween electrodes 90 and 93 and the impedance voltage may be measuredbetween electrodes 91 and 92. This process may continue until impedancemeasurements are calculated for various combinations of electrodes.

The measured impedance voltages may be processed by using a signalprocessor 108 that can extract relevant data from a specific frequencyrange to correlate the impedance change for each electrode to thatelectrode's contact with the target treatment tissue. The impedanceassociated with those electrodes in contact with the tissue will behigher than those that are surrounded by the blood pool. Accordingly,the electrodes with the highest impedance will be the ones in bestcontact with the target treatment tissue, and as a result should providethe orientation of the catheter tip to the treatment tissue site.

FIG. 4B illustrates an embodiment of the thermally conductive region 38of catheter 34 having four electrodes 90, 91, 92 and 94. FIG. 4Cillustrates another embodiment of the thermally conductive region 38 ofcatheter 34 having eight electrodes 90, 91, 92, 93, 94, 95, 96 and 97.The number of electrodes controls the accuracy of the contractassessment of the catheter's thermally conductive region 38. As thenumber of electrodes placed on the catheter's thermally conductiveregion 38 increases, the more accurate the contact assessmentmeasurement. In addition, besides providing contact assessment, thissystem, as well as all the other system embodiments, can also provideenhanced assessment of lesion quality and/or size. For example, bymeasuring the electrical impedance of tissue prior to a cryogenictreatment, and then measuring the electrical impedance of that tissuesubsequent to cryogenic treatment, it is possible to quantify the amountof treated tissue for a particular treatment session or sessions.

Depending on the rate of change of the impedance, the cooling profilemay be adjusted. For example, a cooling profile may be developed for anoptimal treatment regime, where the preset impedances (e.g., Z1, Z2, Z3,and Z4) are desired at corresponding times (e.g., T1, T2, T3 and T4). Asa specific time in the treatment regime is reached, the impedance isdetermined from a measured impedance voltage, and that impedance iscompared to a preset impedance (e.g., Z1, Z2, Z3, and Z4). Depending onthe measured impedance, the cooling profile may be adjusted to increaseor decrease the cooling power of the catheter, thereby providingincreased treatment regime control.

The catheter, as illustrated in FIG. 5 can be used for detecting gas orliquid egress 104 from the catheter. After applying a low frequencyelectrical current (e.g., in the range of 250 Hz to 100 kHz) to the twoelectrodes 90, 92 the impedance can be measured by the impedancemeasurement system 106. The impedance measurement signal is thenprocessed using a signal processor 108 that can extract relevant datafrom a specific frequency range to correlate the impedance change, ifany, due to a gas egress into the blood stream. The application of thelow frequency electrical current causes an electrical field 110 to form,which basically encloses or encompasses the thermally transmissiveregion 38. This electrical field 110 may have similar functionality tothe expandable membrane 64, which is to provide leak detection in theevent that the chamber or inside balloon 62 were to rupture, crack orleak. Thus, the electrical field 110 may be called a “virtual balloon”capable of detecting a fluid egress or expulsion 104 from the catheter34 and generating a signal for automatic shutdown of the catheter system30. Of course, unlike the leak detection of the expandable membrane 64,which measures ruptures or leaks internal to the catheter 34, the leakdetection of electrical field 110 is external to the catheter 34, andfound in the external fluid and tissue being treated. In thosesituations where fluid egress is present, a signal is generated to stopthe flow of cryogenic fluid to the catheter and evacuate all fluid fromthe catheter.

In an alternative embodiment, additional electrodes 86 and 88 may beplaced on the shaft of the catheter treatment section (similar to thoseshown in FIG. 2) to increase the sensitivity of the detection system,and thus provide for lower amplitude and/or lower frequency excitationsignals to be used.

In conjunction with the various electrode configurations describedabove, there are various processing algorithms that may be employed. Asillustrated in FIGS. 6, 7 and 8, processing algorithms for determiningfluid egress, pulmonary vein occlusion/fluid egress verification and tipice coverage/lesion quality measurement are provided.

Referring to FIG. 6, an exemplary fluid egress algorithm is illustrated.The process starts at step 600 and a baseline impedance Z₀ is measuredacross the measurement electrodes (step 610). If the start button forthe treatment cycle has not been activated, then another baseline lineimpedance Z₀ may be measured. Upon the activation of the start button(step 620), the refrigerant is delivered to the catheter (step 630).After delivery of the refrigerant (coolant) (step 630), the impedanceZ(t) (step 640) is measured and a delta impedance (D) Z(t) is calculatedwherein the delta Z is the impedance Z at time t minus the baselineimpedance Z₀ (step 650), DZ(t)=Z(t)−Z₀. In general, the baselineimpedance Z₀ will increase at a linear rate as freezing of the treatmenttissue occurs. For example, if Z₀ is first measured to be 20 ohms, andafter application of a treatment cycle, the impedance Z is 25 ohms, thendelta impedance DZ(t) is 5 ohms. However, if a fluid leak should occur,then the speed of the change with increase rapidly and cause a suddenspike in the impedance Z(t). The spike on the impedance graph shown inFIG. 10 illustrates this situation. In step 660, the signal changes willtypical stop and the first derivative dZ(t)/dt is measured. If thedZ(t)/dt is greater than the threshold value, which is a predeterminedvalue for each catheter, then the freezing cycle is halted (step 670).Otherwise, the system checks to determine if the treatment cycle hasreached the end of the freezing cycle (step 680). If so, then thefreezing is stopped (step 685) and the system is vented (step 690).After the system is vented (step 690), a notification is generated tothe user (step 690).

Referring to FIG. 7, an exemplary balloon catheter controller algorithmfor measuring pulmonary vein occlusion/fluid egress/ice coverage isillustrated. Steps 700 through 720 relate to assessing the quality of apulmonary vein (PV) occlusion. The process commences at step 700 and alow frequency excitation current may be applied across a pair ofelectrodes (step 705). The impedance Z(t) is measured (step 710) and thecatheter is repositioned until the impedance Z(t) is at a maximum, wherethe Z_(max) may be displayed and serves to indicate that a best possiblepulmonary vein occlusion has occurred. Upon the activation of the startbutton (step 725), a high frequency excitation current may be appliedacross a pair of electrodes (step 730), and the baseline line impedanceZ₀ may be measured (step 735). The change to a high frequency excitationcurrent facilitates the measurement of the bio-impedance across atreatment tissue site.

After delivery of a refrigerant (coolant) to the catheter (step 740),the impedance Z(t) (step 745) is measured and a delta impedance (D) Z(t)is calculated wherein the delta Z is the impedance Z at time t minus thebaseline impedance Z₀. (step 735) DZ(t)=Z(t)−Z₀. In step 745, the signalchanges will typical stop and the first derivative dZ(t)/dt iscalculated. If the dZ(t)/dt is greater than the threshold value, whichis a predetermined value for each catheter, then the freezing cycle ishalted (step 750). Otherwise, the system continues the freezingtreatment (step 755) and checks to determine if the impedance Z(t) is ata maximum value (step 760). A graph of the general impedance Z(t) withrespect to time is illustrated in FIG. 9. If the impedance Z(t) is at amaximum value, full ice coverage of catheter treatment tip has occurredand the PV has been occluded. If Z(t) is not at a maximum value, thenthe effectiveness of the treatment is undetermined (step 770). If thetreatment cycle has reached the end of the freeing cycle (step 775),then the freezing is stopped (step 780) and the system is vented (step785). After the system is vented (step 785), a notification is generatedand sent to the user (step 790). Otherwise, the system will measure anew impedance Z(t) (step 745) and calculate a delta impedance (D) Z(t)wherein the delta Z is the impedance Z at time t minus the baselineimpedance Z₀. In step 745, the signal changes will typical stop and thefirst derivative dZ(t)/dt is calculated. The process continues asdiscussed in steps 745 through 790 until the selected treatment iscompleted.

Referring to FIG. 8, an exemplary catheter controller algorithm formeasuring catheter tip ice coverage/lesion quality is illustrated. Theprocess commences at step 800 and a low frequency excitation current maybe applied across a pair of electrodes (step 805). A baseline impedanceZ₀₁ is measured (step 810). Upon the activation of the start button(step 815), the impedance Z(t) is measured. The delta impedance DZ(t) iscalculated wherein the delta Z is the impedance Z at time t minus thefirst baseline impedance Z₀₁ (step 825). DZ(t)=Z(t)−Z₀₁. the value ofdelta impedance DZ(t) may be displayed (step 830). A refrigerant(coolant) is delivered to the catheter (step 835), The value of DZ(t) isprocessed to determine if DZ(t) has reached a saturated condition (step840). If so, the catheter tip is covered with ice (step 845) and thetime to saturation is compared to the time threshold (step 850). If not,a recommendation that the freezing procedure be halted (step 855), thecatheter be repositioned (step 860) and the process restarted (step 800)is generated. If time to saturation is greater than the time threshold,a recommendation that the freezing procedure be halted (step 855), thecatheter be repositioned (step 860) and the process restarted (step 800)is generated. If the time to saturation is less than the time threshold,then maximum freezing capability is requested (step 870).

If the treatment cycle has reached the end of the freeing cycle (step870), then the freezing is stopped (step 875) and the temperature of thecooling segment is measured and compared to a temperature threshold(e.g., +1 degree C.) to determine if the catheter tip has warmedsufficiently to be removed from the tissue treatment site (step 880). Ifthe temperature of the cooling segment is less than the temperaturethreshold (e.g., +1 degree C.), the catheter usually remains in itscurrent position, and another temperature reading is taken. If thetemperature of the cooling segment is greater than the temperaturethreshold (e.g., +1 degree C.), a new baseline impedance Z₀₂ is measured(step 885) and the lesion quality may be calculated (step 890) by theequation: lesion quality=K*(Z₀₂−Z₀₁); where K is a constant multiplierdetermined from in vitro testing or finite element model calculationsand may have a specific value per different catheter type (for e.g., a 6mm long tip will usually have a different K than a 4 mm long tip).

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope andspirit of the invention, which is limited only by the following claims.

1. A method for assessing ice coverage of a catheter treatment tipcomprising: positioning a catheter at a tissue treatment site, thecatheter including a first electrode and a second electrode; applying anelectrical current between the first and second electrode; measuring afirst impedance voltage between the first and second electrodes;measuring a second impedance voltage between the first and secondelectrodes; calculating a difference between the first and secondmeasured impedance values; and assessing ice coverage of the cathetertreatment tip based at least in part upon the calculated difference. 2.The method according to claim 1, further comprising repositioning thecatheter when the calculated difference between the first and secondmeasured impedance values is greater than a predetermined value.
 3. Themethod according to claim 1, wherein the predetermined value isapproximately
 0. 4. The method according to claim 1, further comprisingdelivering coolant to the catheter treatment tip.
 5. The methodaccording to claim 4, further comprising defining a saturation timethreshold for the calculated difference to reach a predetermined value.6. The method according to claim 5, further comprising determiningwhether the calculated difference has reached the predetermined valuewithin the defined saturation time threshold; and terminating coolantdelivery to the treatment tip based at least in part upon thedetermination.
 7. The method according to claim 6, further comprisingrepositioning the catheter treatment tip.
 8. The method according toclaim 7, further comprising reinitiating coolant delivery to thetreatment tip.
 9. The method according to claim 1, further comprisingmeasuring a temperature of the treatment tip, and removing the catheterfrom the treatment site when the measured temperature has reached apredetermined value.
 10. A method for assessing ice coverage of acatheter treatment tip comprising: positioning a catheter at a tissuetreatment site, the catheter including a first electrode and a secondelectrode; delivering coolant to the catheter treatment tip; applying anelectrical current between the first and second electrode; monitoringimpedance voltage between the first and second electrodes; and assessingice coverage of the catheter treatment tip based at least in part uponthe monitored impedance voltage.
 11. The method according to claim 10,further comprising determining that the impedance voltage has reached amaximum value; and correlating the determination to a condition ofsubstantially full ice coverage of the treatment tip.
 12. The methodaccording to claim 10, further comprising defining a saturation timethreshold for the monitored impedance voltage to reach a maximum value.13. The method according to claim 12, further comprising determiningwhether the monitored impedance voltage has reached a maximum valuewithin the defined saturation time threshold; and terminating coolantdelivery to the treatment tip based at least in part upon thedetermination.
 14. The method according to claim 13, further comprisingrepositioning the catheter treatment tip.
 15. The method according toclaim 14, further comprising reinitiating coolant delivery to thetreatment tip.
 16. The method according to claim 10, further comprisingmeasuring a temperature of the treatment tip, and removing the catheterfrom the treatment site when the measured temperature has reached apredetermined value.